Rotating electric machines include a rotor which rotates about a rotating shaft, a stator disposed opposite to a circumferential surface of the rotor and with a stator coil winded around a stator core, and a housing which fixes the stator and rotatably holds the rotor.
When a rotating electric machine is operated as an electric motor, an AC current is fed to the stator coil and a rotating magnetic field is generated, thereby giving turning force to the rotor to obtain mechanical output. Meanwhile, when a rotating electric machine is operated as a generator, turning force is externally given to the rotor to allow the rotor to rotate, thereby obtaining electric output generated at the stator coil.
When a rotating electric machine is operated as an electric motor or a generator in this manner to obtain mechanical output or electric output, the stator coil and the stator core generate heat due to loss of the rotating electric machine. An insulating material used for the rotating electric machine has an upper limit temperature that allows for maintaining insulating performance. Therefore, in order to maintain the temperature of the insulating material of the rotating electric machine to be less than or equal to the upper limit temperature, it is required to cool the rotating electric machine by some method when the rotating electric machine is operated.
Classifying cooling methods of the rotating electric machine by a medium used for cooling, the methods are divided into gas cooling methods using gas such as the air or hydrogen as a cooling medium and liquid cooling methods using liquid such as cooling water or cooling oil as a cooling medium.
Of the above, the liquid cooling methods can be classified into indirect cooling methods to indirectly cool a stator core and a stator coil by cooling a housing by circulating cooling liquid in a passage included in the housing and direct cooling methods to cool a stator core or a stator coil that is a portion generating heat by bringing cooling liquid, such as cooling oil having electric insulation property, into direct contact with the stator core or the stator coil.
It is known that in the indirect cooling methods a passage structure of the cooling liquid significantly influence cooling performance. For example, PTL 1 discloses a passage structure of a housing that allows for suppressing deterioration of cooling efficiency due to stagnation of cooling water.